Vigorous T-cell responses could be mounted against "non-self" while such responses are tightly controlled and tuned to prevent potentially harmful destruction through immune regulatory molecules. We will explore therapeutic potentials of a recently identified co-signal pathway, which includes two ligands called B7-DC and B7-H1 as well as two receptors, a coinhibitory programmed death-1 (PD-1) and a yet unidentified costimulatory receptor (CoR). By computational molecular modeling and site-directed mutagenesis, we have successfully isolated B7-DC mutants (B7-DC*), which lose binding capacity to PD-1 but retain costimulatory function. Furthermore we have also generated blocking mAb to PD-1 with capacity to enhance T cell responses. In this proposal, we will test the hypothesis that selective stimulation through CoR and inhibition of PD-1 will enhance therapeutic tumor immunity. We will first evaluate the role of B7-DC* costimulation in eliciting therapeutic immunity against poorly immunogenic cancers. In addition, B7-DC* will be further engineered to increase its avidity and will be tested for its synergistic effect with B7-1 costimulation as well as underlying mechanisms. To develop PD-1 blockade approach, PD-1 mAb will be tested for its potential in the induction of autoimmune diseases during cancer therapy and new strategies will be tested by combining CD 137 and B7-DC* costimulation to further enhance tumor immunity and eliminate autoimmunity. We will also validate our recent finding implicating that PD-1 expression on lymphoma is beneficial for the generation of immunity. Finally, we will develop and characterize human B7-DC* and PD-1 mAb as a first step towards translation of laboratory study to clinic. Our studies will form the foundation for new approaches towards immunotherapy of cancers.